Page:The American Cyclopædia (1879) Volume XIII.djvu/619

 PLATINUM 599 ladium, rhodium, iridium, osmium, and ruthe- nium, and a little iron, in the form of small flattened grains ; sometimes in larger nodules, alloyed with gold and traces of silver, and with copper, iron, and lead. It is found in alluvial districts, in the debris of the earliest volcanic rocks, on the slopes of the Ural mountains in Russia, in Brazil, Santo Domingo, Borneo, Ceylon, California, and Australia. In Nizhni Tagilsk, Ural mountains, it has been found with chromite in serpentine. Russia affords annually about 800 cwts. of platinum, which is nearly ten times the amount derived from Brazil, Colombia, Santo Domingo, and Borneo ; the yield from Borneo is from 600 to 800 Ibs. annually. It has been found in the sands of the Rhine ; at St. Aray, valley of the Drac, in the French Alps ; in the county of Wicklow, Ireland ; in Honduras ; in Rutherford co., North Carolina ; and at St. Francois Beauce, in the province of Quebec. Although almost always in small grains, it has been found in masses of considerable size ; one weighing 1,088 grains, and having a specific gravity of 18-94, was brought by Humboldt from South America and deposited in the Berlin museum. In 1822 a specimen from Condoto was placed in the Madrid museum which was 2 in. in diameter and weighed 11,641 grains. A mass was found in 1827 in the Ural which weighed 10 T 9 ^ Russian pounds, or 11-57 Ibs. troy. The largest mass ever found weighs 21 Ibs. troy, and is in the Demidoff cabinet. The grains of native platinum usually contain from 75 to 85 per cent, of the pure metal. The extraction of platinum is somewhat difficult, the ordinary method being that devised by Wollaston, which is as follows. The lighter impurities are sep- arated by washing, and the washed ore is first digested in nitric and then in hydrochloric acid, to remove the more easily oxidizable metals. It is then digested at a moderate heat in nitro-muriatic acid containing an excess of hydrochloric acid, and slightly diluted in order to dissolve as little iridium as possible. The yellowish red acid solution is then decanted and treated with sal ammoniac dissolved in five times its weight of water, which precip- itates the metal in the form of a yellow dou- ble salt, ammonium platino-chloride, 2H4NC1, PtCU, which, being washed in cold water, dried, and heated to redness, is reduced to the metallic state as a spongy mass. It cannot be fused into a compact mass by a furnace heat alone, but as it possesses the property of weld- ing like iron, it may be reduced to a compact state when hot by hammering, which is a part of Wollaston's process. Deville and Debray employ fusion in a lime crucible by the oxyhy- drogen blowpipe, to remove traces of osmium and silicon. They have also introduced an entirely new method of extraction, employing fusion instead of the wet way. A small re- verberatory furnace, having a cup-shaped bed of fire brick lined with clay, is heated to full redness and charged with a mixture of equal parts of platinum ore and galena, introduced in small quantities and stirred with an iron rod, while a little glass is thrown in to act as a flux, and afterward by degrees a quantity of litharge equal to that of the galena used. In this way the sulphur is oxidized and expelled, and the lead of the galena and litharge forms a fusible alloy with the platinum. The melt- ed mass is now left to stand till the osmium and iridium fall to the bottom, they not being alloyed by the lead, when the lead and pla- tinum alloy is cautiously taken out with iron ladles, and submitted to cupellation in the or- dinary manner, after which the crude cupelled platinum is refined by the oxyhydrogen blow- pipe on a bed of lime. The alloy of platinum, iridium, and rhodium is harder and withstands a higher heat than pure platinum, and for that reason is better adapted for making crucibles. The symbol of platinum is Pt; its atomic weight, 106-5 ; specific gravity, 21-5. It resists the highest heat of the forge, but melts in the voltaic arc and before the oxyhydrogen blow- pipe, and may be volatilized with scintillations. Its crystalline form as found native is that of the octahedron, but all attempts to produce artificial crystals have failed. It expands less by heat than any other metal, and is much infe- rior to silver, gold, and copper as a conductor of electricity, ranking near iron, as is shown by the readiness with which a small wire is ignited by the galvanic current ; this property is made use of in firing explosive compounds. It does not oxidize in the air at any tempera- ture, nor is it attacked by any one acid ; but if heated to redness in the air in contact with caustic alkalies or alkaline earths, a hydrated oxide is formed which combines with the alkaline base, in a similar manner to palladi- um. Platinum possesses the property of caus- ing the union of oxygen with hydrogen and other combustible gases, even in the compact form, but more highly in the spongy state, and still more so as platinum black. The metal may be obtained in this latter form in several ways, of which the following is one of the most convenient. A solution of chloride of platinum is boiled with an excess of carbonate of soda, to which a quantity of sugar has been added, until the resulting precipitate becomes black. Chloride of sodium is formed, water and carbonic acid are produced by oxidation of the sugar, and the platinum is precipitated, so finely divided that it appears black. This powder is then collected on a filter, washed, and dried by a gentle heat, when it is found to have the power of condensing gases, es- pecially oxygen, in its pores to a remarkable extent. It almost instantaneously converts alcohol into acetic acid, often with sufficient rise of temperature to cause combustion. It also converts wood spirit into formic acid. The unalterability of platinum at high temper- atures, and its power of resisting the action of most chemical agents, render it useful for crucibles, evaporating dishes, forceps and foil